Metallicity of Red Giants in the Galactic Bulge from Near-Infrared Spectroscopy
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We present moderate-resolution (/ = 1300-4800) K-band spectra of more than 110 M giants in Galactic bulge fields interior to -4 (560 pc) and as close as within 0.2 (28 pc) of the Galactic center. From the equivalent widths of three features in these spectra, EW(Na), EW(Ca), and EW(CO) we calculate metallicities for the stars with a new calibration derived by Stephens et al. from globular clusters. The mean metallicity for each field is in good agreement with the recent results from Frogel et al. based on the slope of the giant branch method. We find no evidence for a metallicity gradient along the minor or major axes of the inner bulge (R < 560 pc). The lack of a metallicity gradient in the inner bulge is not predicted by a theoretical model developed by Moll et al. for a bulge formed by dissipative collapse. A metallicity gradient along the minor axis found by Frogel et al. only arises when fields located at larger galactic radius are included. However, these more distant fields are located outside of the infrared bulge defined by the COBE/DIRBE observations. We compute the metallicity distribution for the inner bulge and find a mean value of -0.21 dex with a dispersion of 0.30 dex, close to the values found in 1996 by Sadler et al. for Baade's window. Unlike the absence of a gradient, these values are close to theoretical predictions for a bulge formed by dissipative collapse.
